Tools



TOOLS Filed Nov. 7, 1956 :the: workpiece being `seen :in Figure 4.V

`from Figure 4, seen Ainthe `same Unid Se@ Patent Oi Elmer 0. Thompson,Meadowbrook, Pa., assigner to Philco Corporation, Philadelphia, Pa., avcorporation ofPennsylvania Y b Application November 7, 1956, SerialNo.`620,8438 f.

l 4claims. (culas-,16) A This invention relates to` a-tool for`.producinglplaneparallel wafers, blanks or plates.frombingotslofgermanium, silicon,,-quartz,"glass` and `the like".

Very thin plates or blanks are often requiredlflnaddi- Y tion it isusually desirable to make the b cut Vorfkerf as narrowas possible, sincethe ingots areoften made of highly,l refined and costly material.`In"an-"attempt to l Lkerffl still tended to be lwide and itwasrelativelyrough.

.The plates cut from the ingot had tobeunduly thick and such plates werestill 'subject` tofdanger of breakage,

dliringlthe" cutting. `Ingot material as well as `cutter material waswasted; and variousother problems were encountered.-

3b] It: "is a general object of `this invention to. minimize "toirriprove'thefabrication` ofthin crystal wafers and `the 'like b`y`a`novel abrasive `wire saw design,- .or sawing method,.employed inlieu-,of the aforementioned cutters.

L j Former' Vattempts to` employ abrasive saw cutting, ln' lieu of theabrasive blade or disc` cutting conventional in this field, had4 led todiculties' which were even more severe than those of the cutter` bladesor discs,

as will be explained-hereinafter. i Y `All of the diiculties mentionedhave now been over- "clome, .by providing whatmay generally be called acorn,-

pound motion of an abrasive wire saw, as will beiunderstood readily upona` study of the disclosure which follows.

In the drawing Figure 1 is a somewhat;` schematic side `view of aVmachine incorp'ora'tiligW the present invention.

Figure 2 is an enlarged detail view taken along" line "2`2 of Figure l.i Figure 3 is an enlarged representation of a 'tool and workpiecedetail, seen in the' same view as in Figure 1;

.in section alorigl4 line 3 3 t Figure `4 is a` view taken alonglne4-4in Figure 3 and` further enlarged; and t t view as invthat ligurefandidentifed in` that ligure by number `5.

In Figure l, the saw. 10, for the slicing 3 of a crystal 111, is shownas a wire, `held extended .by` chuck members 12, 13 at opposite ends ofa hacksaw frame 14. `The wire has an` abrasive surface, which` may begenerally cylindrical," although it may have `"suitableland and grooveareas as is knownl inthe abrasive art and not shown herein. Eachsawchuck'mernber canfbe'revolved m about an axis defined bythe wire#Suchrotation may be eiected for instance'by a simple pawl and spoke`wheel mechanism, shown in Figures 1 and 2 as comprising a chuck' collar15 'rotatable in a `bearing 16at `one endofwthe--franie 14; `thisIcollar Lextendingthrough the bearing andihaving astar wheel 5117mountedthereon, on the other s idejof the"` frame 14. 'The `spokes 18 ofthis wheel aretso disposed, relative to a deilector or pawl `19 on astationary frame 20, that a ,terminalpart of each reciprocationothesaw,.win one Patented vJuly as, tsss direction,causes one of the spokes18 to ride up an inclined front surface of the deflector. This resultslin `an angular deflection of the spoke 18, wheel 17, collar 16 nandchuck 15; and al1 such deflections: can be performed, for instance, in acounterclockwise direction in Figure 2, "asshown by arrow 21. Deflectionof the opposite spoke wheel 22 bythe opposite pawl 23 is then performedin the same angular direction and to the same angular extent, accordingto the present arrangement, so that opposite ends of the saw 10 arealternately uniformly` twisted by `small angles, resulting in gradual,complete, bodily rotation of the `reciprocating saw. The deilectors orpaw1si19, 23 may allow overriding by the spoke wheel cooperatingtherewith; reverse actuation of the spoke wheel, `on each return stroke,can be avoided. This can be done for instanceby mounting each deector ona pivot 24, in association with a stop member 25, so that thedeectorisfree on `each return `strvoketo idlewhile rising from the positionshown, but that it causes positive, predetermined deflection of thespoke wheel on eachforward stroke. There may be used, ffor instance,sixteen spokes on each wheel 17, 22, resulting in a gradual, completerotation of saw 10 during every sixteen forward and backward strokes. t

As indicated inFigure 5 the abrasive surface of wire 10 is in broadcontact with and exerts lateral as well as downward pressure against theworkpiece. This pressure is vibratory. In order to `successfully cut ina thin wafer of material from the crystal ingot 10, the saw must applythe least possible lateral pressure against the gthin, semi-attachedslab 26, exposed on the outside of `the kerf 27. Therefore the danger ofbreakage of thinV slabs, in process of formation, is reduced byminimizing the mass of vibratory saw structure, in Contact with thisslab. This, in turn, is a reason why it is preferable, for

the cutting of thin wafers, to utilize thin wire saws. Such a saw maycomprise for instance, as shown in Figure 5, a metallic core filament28, with an abrasive body 29 coaxially bonded thereto. The core 28 mayfor example consistV of high tensile strength drawn tungsten; and atine-grit diamond-bearing coating may be bonded to this Acore.

consistently encountered so long as the same saw was used.

The deection was often as great as one-quarter of an inch in the cuttingof a crystal of a few inches thickness, when the saw tension was normalor moderate; it was at any rate the equivalent of many times the desired`slab thickness of a few mils. Thus the plates, heretofore `produced bya saw, could be made about as thin as the saw itself; their surfaceswere suiciently parallel; but

they were non-planar to a major extent.

Such plates were practically useless where planeparallel wafers wererequired; and while subsequent straightening of the curved surfacescould be achieved on relatively thick wafers or plates, by known etchingprocesses, this would usually'involve prohibitive costs, even ifetched-olf` material `be recovered. `The curved lateral deilectionof thesaw has now been avoided, for all practical purposes presentlyconsidered. This has `been, achieved by the progressive, gradualrotation of the saw wire, elfected by a spoke wheel and deflectormechanism 18, 19 or the like. b

kmade once more to Figure 1.

v Itappears-although I do Nnot wish to be bound toany theorythat in theabrasive surface portion ofthe saw, sliding over the crystal, individualdiamonds tend to cut Ain vastly different, individual directions 30X,SQY, ctc.,

Figure 5, and that said abrasive areaacadrdingly; may be biased into aparticular average direction 3 0Z= more or less diiferent from thevertical. The successive exposures of different abrasive areas, broughtabout in predetermined ways by the deiector mechanism '18, 19 (not=merely left to the abrasive wire and the crystal surfaces themselves),seem to have the effect that the different deviations of said.directions 30Z, from a vertical line, will mutually compensate for oneanother.

. This seems to explain the consistent, planar, vertical cutting, whichI have obtained with this saw. It Vfurther seems to be desirable, forthis directional compensation,

that means or, operations are provided whereby the psition o f theabrasive wire is shifted incidentl to the more basically required sawmovements.

VWhile the details of theillustrated case, relating to thisshifting'incident to basic movement, are subject to variation, they haveseveral advantageous features. yIn the first place, the operation asdescribed involves partly overlapping steps of longitudinalreciprocation and uni,- directional rotation; this contributes to speedycutting. In addition, twisting is applied to each end portion of thewire. This is done, more particularly, at a moment when the length ofsuch end portion, unconiined between work- -piece 11 and chuck 12 or 13and unrestrained for torsional response, is at a maximum; this adds tothe safety and smoothness of said response.

I have successfully cut plane-parallel germanium wafers of eightmilsthickness with a tolerance of not more than d Vhalf a mil, fromcrystals of about ltwo inches diameter.

Such cutting was heretofore impossible. I cut each of said wafers byseveral thousand reciprocations of a line grit diamond wire saw havingabout seven inches length and-at least initiallyabout ten mils effectivediameter, revolving once every sixteen reciprocations. I have rapidlyproduced large numbers of `said wafers by each saw wire, constructed andoperating as described.

For some added mechanical features, reference may be It will be seenthat the reciprocating saw frame 14 is adapted to slidel ina guideportion 31 of the fixed frame 20, in a direction along the axis of thesaw wire 10. The frame 14 is shifted back and forth by a motor 3'2,through the intermediary of a crank 33 and pitman 34; the motor beingmounted on a bracket 35 which may be rigid with the frame 20.

The frame 20 may rock on a horizontal pivot pin 36 held by a suitablesupport 37, so that successive reciprocations of the saw occur in asingle vertical plane or in` other words, so that the reciprocating sawcan gradually move into the crystal 11, transversely of wire 10. Thislatter movement may be controlled by a screw mechanism 38, 39, actuatedby a separate motor through a worm and worm gear drive 41, 42; and sucha mechanisrn may be connected with the frame 2Q by pivot means Thecrystal 11 may be mounted on a ceramic base 44 horizontally slidable ona support 45; and suitable means, not shown, may be provided forcontrolling such sliding and for receiving the cut-off wafer at the endof each saw operation.

Coolant liquid may be applied to the working area of the saw 10 througha dispensing nozzle 46 to submerge the operating portion of the-saw 10,asbest shown in Figures 3 and 5, where the liquid appears at 47. Wastecoolant liquid can be intercepted inV a pan' 48, Figures` 3 and 4. Therelatively small amounts of germanium,

vpresent in such waste liquid as a result, of the use of the In formercrystal-slicing processes, by contrast, enormous amounts' of germaniumor the like were wasted. The cutting discs, in order to minimizevibration, had t0 be at least about fifteen to twenty mils thick incases where the present saw can be eight mils thick; this obviouslyinvolved the making Vof a kerf of two to three times the thickness ofthe wafer. In addition the wafers produced by such twenty mil discshad`to be atleast about twenty mils thick, themselves, because ofsurface roughness andof persisting dangers of breakage by the discs;thus involving the necessity of lapping off and etching off more thanhalf of the sliced, raw wafer. Recovery of the large amounts-ofgermanium waste, from solutions fullof etching, lapping and disc sawbinder Arnaterials, and the like, was much less efficient and much' moreexpensive, even on :gram by gram basis, than the recovery of the smallamounts of kerf waste, from the present coolant solution.

It has further been found that by virtue of the present slicer, lappingof the wafer can be omitted entirely; and most or all of' said formeretching-mainly the .so-called sizing etch-can also be omitted, in mostfabrication methods using the present tool.

' Still another advantage of the present tool resides in the fact that-it allows the growing and use of crystals of practically unlimitedsize, whereas the former disc cutters were limited to small crystaldiameters such as two inches. For a number of known reasons, which neednot bedeveloped here, the growing and use of larger crystals is greatlypreferred.

While only a single embodiment of the invention has comprises;performing a primary movement of the wire alone the 'axis thereof; andapplying a farce t0 the wire to perform secondary movements thereof,alternately at two ends of the wire, each secondary movement beingperformed at one end while a reciprocation is being completed.`

3. The method of making flat, thin wafers which comprises: reciprocatingthe ends of a thin abrasive wire, tensioned over a workpiece, alongsuccessive lines substantially parallel to the wire and which lie in asingle plane; and, incident to such reciprocating, applying forces towire. portions not contacting the workpiece, for twisting such portionsangularly about the axis of the wire, whereby deviation ofy the saw kerffrom said plane is minimized.

4. In a saw: a thin abrasive wire; holding devices, one for holding eachend of the wire; means spacing the holding devices from oney another fortensioning the wire over a workpiece; means for reciprocating thespacing means to move the holding devices toward and away from theworkpiece and thereby to saw a kerf into the workpiece; and means forturning such holding devices, incident to such reciprocating, so asy totwist the wire portions extending between the holding devices and theworkpiece.

References Cited in the file of this patent Y UNITED STATES PATENTS125,804 Gear Apr. 16, 1872 1,876,480 Weinhelz et al Sept. 6, 19.322,328,998 Radford Sept. 7, 19.43 2,479,929 Harris Aug. 23, 1949 vFOREIGN PATENTS 15,931 'switzerland De.`4, 1891

